When a broadband radio transmitter, such as a 700 MHz (Megahertz) Broadband Long Term Evolution (LTE) transmitter, is operating in the vicinity of a narrowband radio receiver, such as a Public Safety (PS) narrowband receiver, out-of-band emissions (OoBE) of the broadband transmitter may cause considerable interference to the narrowband PS receiver. The broadband transmitter's OoBE will sum with the noise of the receiver, resulting a decrease in a signal to interference-plus-noise ratio (SINR) at the narrowband PS receiver and thereby desensitize the receiver.
For example, FIG. 1 is an exemplary spectral graph 100 depicting a broadband signal 102 whose frequency band 108 is in close proximity to the frequency band 112 of a narrowband signal 106. Despite the inclusion of a guard band 110 as a buffer between the broadband signal and adjacent signals, such as narrowband signal 106, an OoBE 104 of broadband signal 102 still spills into the bandwidth of narrowband signal 106, resulting in receiver desensitization, that is, reduced Signal-to-Noise Ratio (SNR) 114 at a narrowband receiver.
For example, such receiver desensitization is known to occur in cases such as the C band, where the close proximity of the C block uplink (transmit) band to the Public Safety Narrowband (receive) band causes desensitization of a narrowband receiver when in close proximity to a C band uplink transmitter. More specifically, in the 700-800 MHz band, the 1 MHz guard band separating the C band uplink (776-787 MHz) from the adjacent Public Safety Narrowband (PSNB) (769-775 MHz) may fail to adequately protect PSNB transmissions from interference from a nearby C band transmitter. Radio Frequency (RF) filters can only provide limited protection from interference in the PSNB by the C band uplink transmissions and cannot eliminate such interference entirely. As narrowband PS radios are used primarily for voice transmissions and are considered to be mission-critical, little degradation of performance can be tolerated.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and/or relative positioning of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention. It will further be appreciated that certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required. Those skilled in the art will further recognize that references to specific implementation embodiments such as “circuitry” may equally be accomplished via replacement with software instruction executions either on general purpose computing apparatus (e.g., CPU) or specialized processing apparatus (e.g., DSP). It will also be understood that the terms and expressions used herein have the ordinary technical meaning as is accorded to such terms and expressions by persons skilled in the technical field as set forth above except where different specific meanings have otherwise been set forth herein.